This invention relates to a method and a punch/die assembly for use in the production of heat exchanger fins as in, for example, air conditioners for rooms and motor vehicles, and, in particular, to a method which does not require any preliminary punching prior to the regular punching indispensable to the conventional manufacturing process of said fins, which contributes to remarkable improvement of productive efficiency and quality of products.
In general, the cooling fins are constituted by a number of superposed square sheets each having a multiple number of flanged apertures in registration with similar flanged apertures in the overlying and underlyig sheets, and a number of copper tubes passed through the respective flanged apertures. The flanges are usually turned outwardly to form beads around the marginal edges of the apertures for the purpose of maintaining a predetermined distance between the adjacent apertures and at the same time for reinforcing purposes. Therefore, the flanges are required to have at least projecting and curling height by more than 1.8 mm.
Referring to FIG. 1 of the accompanying drawings, in forming a flanged aperture 14 in an aluminum sheet 10, it has been the conventional practice to perforate in the first step an aperture 12 which has a diameter far smaller than that of a flanged aperture 14 to be ultimately formed, pressing the marginal edge portions upwardly by means of a punch 13 thereby to form the flanged aperture 14 of the predetermined dimension. However, where the aperture 12 is formed in a small diameter in an attempt to increase the height of the ultimate flange, cracking often occurs to the marginal edge portions of the aperture when pressed by the punch. Therefore, without a preliminary treatment or machining, it has been difficult to form a flange which has a height greater than 1.8 mm.
The pre-machining usually includes pressing of the coiled aluminum material 10 by a punch to form a bonnet-like recess 15 of a diameter far larger than that of the intended flanged aperture, as shown in FIG. 2, and further pressing of the recessed portion 15 by another punch to reduce its diameter while increasing its height as shown particularly in FIG. 3. These operations are repeated to obtain a number of flanged apertures 14 of the predetermined diameter and height as shown in FIG. 1. This method is generally referred to as "drawing" and is capable of forming a flange of a relatively great height by the gradual or progressive stretching of the coiled aluminum material 10. However, the just-mentioned method has inherent drawbacks in that the circumferencial wall of the flanged aperture 14 bears concentric hammered marks as a result of the repeated punching operation and wrinkles appear at the both ends of the coiled aluminum material 10 to cause warping or distortion to the fins as a whole.